I can't add an Ticket to an Sprint.
I put it via drag and drop an the ticket dropped in the sprint list, but if i refresh, it's not saved and the ticket is in the backlog again.
This sounds like a bug with the Next-gen projects. Is this affecting all issue types or only some? Does turning the Sprint feature off and back on from the Features setting page fix this problem? If you are still experiencing the problem, please raise a support ticket here so we can assist you further.